Coating compositions which are curable under the influence of radiation in general, and ultra-violet light and electron beam in particular, are well known. Unfortunately, these coating compositions suffer from a number of disadvantages. For example, many of these coatings have insufficient flexibility that causes them to crack in use when applied to flexible substrates. Other compositions do not adhere sufficiently to the substrate or to laminated layers of the coating with the undesirable result that they become dislodged or peel. Still other coating compositions require the use of solvents that must be evaporated during the curing process. The evaporation of such solvents increases production time, consumes energy and creates atmospheric pollution problems. Other compositions produce coatings that yellow, do not weather well and have insufficient scratch resistance, stain resistance, abrasion and/or solvent resistance.
Still other compositions cannot be applied to polyvinyl chloride (PVC) substrates which are subjected to high use temperatures, since the dehydrohalogenation of the PVC, which takes place at elevated temperatures, deleteriously affects the coating. Additionally, some compositions do not cure adequately with low dosage radiation so as to allow sufficiently rapid processing for advantageous commercial feasibility.
Burlant et al in U.S. Pat. No. 3,509,234 teach radiation curable coating compositions comprising a urethane modified vinyl prepolymer and copolymerizable vinyl monomers suitable for coating wood or metal. Burlant et al, however, do not disclose the use of N-vinyl-2-pyrrolidone or optical stabilizers in their composition.
Lorenz et al in U.S. Pat. No. 4,129,667 teach a radiation curable coating composition comprising a urethane vinyl oligomer and an acrylic ultraviolet light absorber to prevent yellowing, and optionally, copolymerizable vinyl monomers. Lorenz et al, while disclosing the use of N-vinyl-2-pyrrolidone as suitable for use as the optional vinyl monomer therein, teach that this monomer may be incorporated in large amounts, whereas the subject invention teaches the critical use of a limited amount of this monomer which can be included in the composition.